1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device, more particularly, to a method for manufacturing a liquid crystal display device having a thin film diode as a switch element.
2. Description of Related Art
In general, a thin film transistor (TFT) is used as a pixel switch element for an active LCD device, and a TFT exhibits the high on/off current ratio (>106) to display images with improved quality. However, the process for preparing a TFT usually requires five to six masking steps. The process for preparing a TFT is complex and the yield is reduced. Thereby, those skilled in the field focus on reducing the steps and enhancing the yield.
The idea that a thin film diode (TFD) is used as a pixel switch element was brought up in 1983. The process for preparing a TFD requires three masking steps, and thereby the manufacturing process of a TFD is simpler than that of a TFT. A TFD comprises a selective line extending to a pixel area, a pixel electrode overlapped by the selective line, and a semi-insulating layer sandwiched between the selective line and the pixel electrode. The area of the three-layered structure is the area of a TFD. The driving action is performed by applying a voltage to the selective line to turn on the TFD, and thereby the pixel signal can be written to the pixel area. In addition, a black matrix of the color filter covering the TFD area can protect the semi-insulating layer from light that would affect electrical properties of the TFD.
U.S. Pat. No. 5,204,764 discloses that a TFD of a Metal-Insulator-Metal (MIM) structure is used in combination with a dual selective line to drive an LCD device so as to improve the display quality, as shown in FIG. 1. However, the area of the TFD for the TFD display device disclosed by U.S. Pat. No. 5,204,764 has to be increased to make the driven current large enough, and thereby the treatment reduces the aperture ratio of the pixel area. In addition, since the material of the upper metal layer is different from that of the lower metal layer, the current-voltage (I-V) characteristic of the thin film diode is asymmetrical, resulting in a poor display quality.
U.S. Pat. No. 6,243,062 discloses a structure of two back-to-back thin film diodes to improve the I-V symmetry (as shown in FIG. 2). Although the method disclosed by U.S. Pat. No. 6,243,062 can efficiently improve the I-V symmetry, the manufacturing process comprises one more step and the aperture ratio is reduced.
According to one of the conventional methods, a metal layer, a transparent electrode layer, and a semi-insulating layer are coated in sequence, and a selective line is defined by a first masking step; subsequently, a photosensitive polymer insulating film is deposited, and then the TFD area is defined by a second masking step, and a transparent electrode layer is deposited; and then a pixel area and a thin film diode are accomplished by a third masking step, as the structure shown in FIG. 3. The conventional method can resolve the issue of asymmetrical I-V characteristic and enhances the breakdown voltage. However, the process of the conventional method still comprises three masking steps, and thereby the issue of complex processing is not resolved.